Context. As potential progenitors of several exotic phenomena including gravitational wave sources, magnetic stars, and Be stars, close massive binary systems probe a crucial area of the parameter space in massive star evolution. Despite the importance of these systems, large uncertainties regarding the nature and efficiency of the internal mixing mechanisms still exist.Aims. We aim to provide robust observational constraints on the internal mixing processes by spectroscopically analyzing a sample of three massive overcontact binaries at different metallicities.Methods. Using optical phase-resolved spectroscopic data, we performed an atmosphere analysis using more traditional 1D techniques and the most recent 3D techniques. We compared and contrasted the assumptions and results of each technique and investigated how the assumptions affect the final derived atmospheric parameters.Results. We find that in all three cases, both components of a system are highly overluminous, indicating either efficient internal mixing of helium or previous nonconservative mass transfer. However, we do not find strong evidence of the helium or CNO surface abundance changes that are usually associated with mixing. Additionally, we find that in unequal-mass systems, the measured effective temperature and luminosity of the less massive component places it very close to the more massive component on the Hertzsprung-Russell diagram. These results were obtained independently using both of the techniques mentioned above. This suggests that these measurements are robust.Conclusions. The observed discrepancies between the temperature and the surface abundance measurements when compared to theoretical expectations indicate that additional physical mechanisms that have not been accounted for so far may be at play.